The present invention relates generally to communication systems. More particularly, the present invention relates to the decoding of encoded digital communication signals transmitted over a fading channel by generating side (reliability) information at the receiver.
In TDMA (time-dimension multiple access) and other communications system, Rayleigh fading can present significant problems. Reliable communication over fading channels requires a large bit energy to noise ratio             E      b              N      o        .
It is known that when communicating over a fading channel, the uncoded bit error rate (BER) decreases inverse linearly, rather than exponentially, with             E      b              N      o        .
See, for example, Wozencraft et al., Principles of Communication Engineering, John Wiley and Sons (1965). While a desirably low error probability of 10xe2x88x925 can be achieved with a signal margin of only 13.4 dB for a noncoherent channel with no fading using binary orthogonal signaling, a signal margin of approximately 50 dB is required for a fading channel. See, for example, Viterbi et al., xe2x80x9cAdvances in Coding and Modulation for Noncoherent Channels Affected by Fading, Partial-Band, and Multiple-Access Interference,xe2x80x9d Advances in Communications Systems, vol. 4, pp.279-308. Fading can also cause a loss in capacity and a reduced channel cutoff rate, as described in Stark, xe2x80x9cCapacity and Cutoff Rate of Noncoherent FSK with Nonselective Rician Fading,xe2x80x9d IEEE Trans. Commun., vol. COM-33, pp.1036-44 (September 1995).
To compensate for the signal and capacity loss of fading, most communication systems use some form of error-correction coding. For fading channels, most of the loss incurred from fading can be recovered using diversity (repetition) coding with some optimally-selected coding rate. For example, a repetition coding scheme can reduce the required signal margin necessary to achieve an error probability of 10xe2x88x925 from 50 dB to about 22 dB.
In a fading time-selective TDMA communication system, more than one data symbol is transmitted per time slot. If the system uses some form of coding, it is desirable to obtain information concerning the reliability of the symbols in a particular time slot, erase unreliable symbols, and use errors-and-erasures correction decoding. Such reliability information can include, for example, information indicative of the number of errors in a particular transmission, xe2x80x9csoftxe2x80x9d information used to decode the transmitted information, and other types of information. Thus, it is desirable to develop practical techniques for generating reliability information during each time slot.
The most common techniques for obtaining reliability information about a channel for coded communications systems fall generally into two categories: pre-detection techniques and post-detection techniques. Such techniques are described in for example, Pursley, xe2x80x9cPacket Error Probabilities in Frequency-Hop Radio Networks-Coping with Statistical Dependence and Noisy Side Information,xe2x80x9d IEEE Global Telecommun. Conf. Record, vol. 1, pp.165-70, (Sec. 1986). Pre-detection techniques are usually complex, involving methods such as energy detection or channel monitoring, and are therefore undesirable. Among Post-detection techniques, McEliece et al., xe2x80x9cChannels with Block Interference,xe2x80x9d IEEE Transaction on Inform. Theory, vol. IT-30, no. 1 (January 1984) suggested the transmission of test bits to learn about the channel. This method was applied to frequency-hopped multiple access channel to detect the presence of a bit in a given time slot in Pursley, xe2x80x9cTradeoffs between Side Information and Code-Rate in Slow-Frequency Hop Packet Radio Networks,xe2x80x9d Conf. Record, IEEE Int""l. Conf. on Communications (June 1987). Similar techniques have been used to generate reliability information concerning a hop in a frequency-hopping spread-spectrum communication system in the presence of fading, as suggested in Hassan, xe2x80x9cPerformance of a Coded FHSS System in Rayleigh Fading,xe2x80x9d Proceedings of the 1988 Conference on Information Sciences and Systems. Similarly, test bits can be used for carrier recovery and synchronization purposes. All of these methods described above involve making xe2x80x9chardxe2x80x9d decisions on the test bits, resulting in a loss of power. In a conventional hard decision case, the receiver makes hard decisions on the test bits T. If more than a threshold number or percentage of the test bits in a timeslot are in error, then the detector declares all of the data symbols D transmitted during that slot as xe2x80x9cbadxe2x80x9d, and generates erasures for all symbols in the bad slot. If fewer than the threshold number are in error, then the detector declares all symbols transmitted during the slot as xe2x80x9cgoodxe2x80x9d, and delivers the corresponding estimates to the decoder. The performance measure of interest in the hard decision case is the probability of bit error, and the threshold must be chosen to minimize this probability. It would be desirable to reduce power loss in a practical, relatively simple method for generating reliability information.
According to exemplary embodiments of the present invention, side (reliability) information indicative of the reliability of the data transmitted in a time slot in a coded TDMA communication system subject to time-selective Rayleigh fading is generated by performing soft decisions to decode test bits. According to a first method, transmitted test bits known to the receiver are included in each slot, and a mathematical distance, such as the Euclidean or Hamming distance between the transmitted known test bit sequence and the corresponding received sequence, is determined by the receiver to decide whether the corresponding slot is reliable or not reliable. Alternatively, the channel state during a slot interval can be determined in a system which uses concatenated codes. According to this embodiment, the inner code is used to generate the information about the reliability of the data received over a channel. Significant enhancement in system performance, particularly with respect to the signal-to-noise ratio, is possible using the techniques of the present invention.